It isn’t my fault, they configured it that way before I got here! That was the entry level technician’s story Monday morning, and he was sticking to it. 
Here is the rest of the story.   Over the weekend, some testing had been done regarding a proposed BGP configuration.   The objective was simple, R1 and R3 needed to ping each others loobacks at 1.1.1.1 and 3.3.3.3 respectively, with those 2 networks, being carried by BGP.  R2 is performing NAT.    The topology diagram looks like this:

The ping between loopbacks didn’t work, but R1 and R3 had these console messages:

R1#
%TCP-6-BADAUTH: No MD5 digest from 10.0.0.3(179) to 10.0.0.1(28556) (RST)
R1#
%TCP-6-BADAUTH: No MD5 digest from 10.0.0.3(179) to 10.0.0.1(28556) (RST)
R1#

R3#
%TCP-6-BADAUTH: No MD5 digest from 23.0.0.1(179) to 23.0.0.3(59922) (RST)
R3#
%TCP-6-BADAUTH: No MD5 digest from 23.0.0.1(179) to 23.0.0.3(59922) (RST)
R3#

The senior engineer looked at the configurations for R1, R2 and R3 and found 5 specific items, each of which was independently causing a failure.
Here is the challenge:  Can you find 1 or more of them?
Let us know what your troubleshooting skills can find, and post your comments here on the blog.
Here are the configurations for the 3 routers:

R1#show run
version 12.4
hostname R1
!
interface Loopback0
 ip address 1.1.1.1 255.255.255.0
!
interface FastEthernet0/0
 ip address 10.0.0.1 255.255.255.0
!
router ospf 1
 network 10.0.0.0 0.0.0.255 area 0
!
router bgp 1
 no synchronization
 bgp log-neighbor-changes
 network 1.1.1.1 mask 255.255.255.255
 neighbor 10.0.0.3 remote-as 3
 neighbor 10.0.0.3 password cisco
 no auto-summary
!
end
R1#

R2#show run
version 12.4
hostname R2
!
interface Loopback0
 ip address 2.2.2.2 255.255.255.0
!
interface FastEthernet0/0
 ip address 10.0.0.2 255.255.255.0
 ip nat inside
 ip virtual-reassembly
!
interface FastEthernet0/1
 ip address 23.0.0.2 255.255.255.0
 ip nat outside
 ip virtual-reassembly
!
router ospf 1
 network 2.2.2.2 0.0.0.0 area 0
 network 10.0.0.2 0.0.0.0 area 0
 network 23.0.0.2 0.0.0.0 area 0
!
ip nat inside source static 10.0.0.1 23.0.0.1
ip nat outside source static 23.0.0.3 10.0.0.3
!
end

R3#show run
version 12.4
hostname R3
!
interface Loopback0
 ip address 3.3.3.3 255.255.255.0
!
interface FastEthernet0/1
 ip address 23.0.0.3 255.255.255.0
!
router ospf 1
 log-adjacency-changes
 network 23.0.0.0 0.0.0.255 area 0
!
router bgp 3
 no synchronization
 bgp log-neighbor-changes
 network 3.3.3.3 mask 255.255.255.255
 neighbor 23.0.0.1 remote-as 1
 neighbor 23.0.0.1 password cisco123
 no auto-summary
!
end
R3#

Let us know what you find!